In applications such as printed circuit board (PCB) production, a laser may be used to expose a pattern on a photoresist coating on a copper coated substrate. In a typical exposure system such as the DP 100 of LIS of Germany and available from Orbotech Ltd. of Yavne, Israel, a CW UV laser beam is scanned across the PCB surface, while its intensity is modulated in accordance with a raster pattern to be generated. The modulating device receives electronic pixel data supplied by control circuitry. In modern PCB production it is desirable to operate at high data rates to increase production speed. The practical data rate is limited by the modulation rate and/or the available laser power.
In the production of PCBs utilizing UV sensitive photoresists a CW Argon Ion laser is often used. Although they are widely utilized as UV light sources, Argon lasers, being gaseous lasers, have a number of drawbacks such as their being complicated and delicate to operate, poor maintainability and/or high price.
Various methods for producing UV laser radiation are known. For example, one such method utilizes an IR solid state laser oscillator to generate mode locked high repetition rate laser light pulses. The wavelength of the IR mode locked laser light is converted to UV by passing the mode locked IR light pulses through a non linear medium. However, the utilization of such lasers to expose photoresist at high data rates is severely limited by a paradox inherent in the frequency conversion process, which is extremely non-linear. Frequency conversion becomes more efficient as power increases.
Although mode locking is useful to obtain laser pulses each of which have a high peak power as is necessary to promote high efficiency frequency conversion, as the repetition rate of the laser pulses increases, for example in order to achieve increased data rates, peak power in individual pulses decreases and the resulting average UV power drops rapidly. Thus, for a given average IR power, the average peak power after frequency conversion drops as the repetition rate of the laser pulses increases, leading to reduced UV generation efficiency.
Practically, using pulsed lasers to write raster patterns is problematic for various reasons. Modulating data at a rate exactly equal to the pulse repetition rate of a mode locked laser is problematic due to difficulties in high speed pulse and data synchronization. Conversely, modulating data at a rate different than the pulse repetition rate of a mode locked laser is problematic due to timing errors in which a pulse is not available at exactly the time required to write or expose a pixel which is supposed to be written or exposed. This latter problem is particularly prevalent when data rates for writing pixels approach or exceed the pulse rate of an exposing radiation source—such as a mode locked laser.
U.S. Pat. No. 3,447,856 describes an optical pulse multiplier operative to split an incoming pulsed laser beam, to lengthen one of the optical paths with respect to the other and to recombine the optical paths.
GB patent application 2,245,790 A describes a structure comprising parallel plane mirrors operative to generate a plurality of mutually time delayed pulse trains from an incoming pulse train. The mutually time delayed pulse trains are subsequently recombined.
U.S. Pat. No. 5,462,433 describes a device for use in electronic warfare for delaying coherent RF signals with an adjustable time delay element. The RF signals are divided into multiple signal paths, some of the paths are delayed relative to the other paths, and then the delayed and non delayed paths are recombined to increase the pulse repetition rate.
U.S. Pat. No. 4,205,348 describes a laser scanner employing an acousto-optic modulator configured to take advantage of the so-called Scophony effect to simultaneously information modulate and deflect an incident CW laser beam so that the beam tracks a facet of a polygon scanner. The system reduces blurring in images on a recording surface.
An article entitled, “Pulsed Scophony Laser Projection System”, J. B. Lowry et. al., Optics and Laser Technology Vol. 20 No. 5 (October 1988), describes the use of a pulsed laser illumination to achieve a “freezing” effect instead of a scanning motion as is present in conventional Scophony modulators. In the absence of scanning motion in the image plane, the laser pulse repetition rate is both equal to and well synchronized with the data modulation rate.
WO 00/11766 and U.S. Pat. No. 6,037,967, the disclosures of which are incorporated herein by reference, describe direct scanner systems for printed circuits and semiconductor masks, respectively, in which a laser pulse rate that is equal to or higher than the data rate is utilized.